Fasted Cardio: Should You Exercise on an Empty Stomach?

Fasted cardio, the act of exercising on an empty stomach, has become a hot topic in the fitness world.

The idea is that when you hit the treadmill or pavement before breakfast, your body turns to fat stores for energy instead of relying on recently consumed carbohydrates.

This concept has sparked a lively debate among fitness enthusiasts and experts alike. Some swear by fasted cardio, claiming it’s their secret weapon for weight loss.

Others, however, argue that the science doesn’t support the hype, and that fasted cardio could even be counterproductive in some cases.

What is Fasted Cardio?

Fasted cardio is a fitness strategy that involves performing cardiovascular exercises while your body is in a fasted state, meaning without having eaten for a prolonged period (typically 8-12 hours).

The theory behind fasted cardio is based on how your body uses energy.

When you eat food, your body breaks it down into various types of nutrients, including carbohydrates which are then converted into glucose.

This glucose is either used immediately for energy or stored in your muscles and liver as glycogen for later use.

When you exercise after eating, your body uses this glucose and glycogen for energy.

However, when you’re in a fasted state, your body’s glycogen stores are low.

The theory is that in this state if you do cardio, your body has to turn to other energy sources – namely, your stored fat.

So, the idea is that fasted cardio could potentially burn more fat than fed cardio.

Timing and Execution:

  • When: Most commonly done first thing in the morning, before breakfast, but can also be incorporated into intermittent fasting schedules.
  • Duration and Intensity: Generally low-to-moderate intensity cardio for longer durations (30-60 minutes) is recommended for fasted workouts. It can involve any type of cardio exercise – running, cycling, swimming, or using machines like the elliptical or stair climber. The intensity can vary based on personal preference, from low-intensity steady-state (LISS) workouts to high-intensity interval training (HIIT).
  • Hydration: Staying well-hydrated is crucial during fasted cardio to avoid dehydration and maintain optimal performance.
  • Fueling for After: Refueling with a balanced meal or snack post-workout is essential to replenish glycogen stores and aid in recovery.

The Science Behind Fasted Cardio

The theory that fasted cardio leads to greater fat loss is based on the body’s energy utilization process.

  • When you exercise, your body needs energy, and it typically gets this energy from the food you’ve eaten.
  • The carbohydrates in your food are broken down into glucose, which is either used immediately for energy or stored in your muscles and liver as glycogen.
  • When you’re in a fasted state, your body’s glycogen stores are low. So, when you exercise, your body needs to find another source of energy.

The theory is that it turns to your stored fat for this energy, leading to greater fat loss. This is why some people believe that fasted cardio can be a more effective way to lose fat.

However, it’s important to note that while this theory sounds promising, the effectiveness of fasted cardio is not fully supported by scientific research.

Some studies have found that while fasted cardio may increase fat oxidation during exercise, it does not necessarily lead to a greater overall fat loss.

Other research suggests that whether you eat before a workout may not matter as much as the total amount of calories you consume and burn throughout the day.

In other words, while fasted cardio could potentially help some people burn more fat during their workouts, it’s not a magic bullet for weight loss.

A balanced diet and regular exercise are still the most important factors for losing weight and improving your health.

Research Findings

Scientific research has been conducted to investigate the effectiveness of fasted cardio, with mixed results.

Here are some key findings:

2017 Scientific Review by Researchers in Australia

2017 scientific review conducted by researchers in Australia looked into five separate studies involving a total of 96 participants.

The review aimed to measure the effectiveness of exercising after an overnight fast.

The authors determined that working out post-fast had very little—if any—effect on body mass.

This suggests that while fasted cardio may increase fat oxidation during exercise, it does not necessarily lead to a greater overall fat loss.

2018 Review from the University of Limerick in Ireland

In a 2018 review from the University of Limerick in Ireland, researchers examined 46 previously published studies.

They found that when study participants ate before a cardio session, they were able to work out for longer periods of time.

This suggests that pre-exercise feeding may enhance prolonged aerobic performance.

Benefits and Risks of Fasted Cardio

  1. Increased Fat Burning: The main benefit of fasted cardio is increased fat burning. Research shows that aerobic exercise in a fasted state induces higher fat oxidation than in a fed state. This means your body may burn more fat as an energy source during your workout.
  2. Convenience: If you’re tight on time, fasted cardio saves you from having to prepare, eat, and digest a meal beforehand.
  3. Intermittent Fasting: If you practice intermittent fasting, fasted cardio allows you to exercise before you eat for the day.
  4. Comfort: Some people find fasted cardio easier on their digestive system. If you have a sensitive stomach or feel more energetic without a meal before a workout, fasted cardio could be an effective option.

Potential Risks and Safety Considerations

While fasted cardio may have some benefits, it’s important to be aware of the potential risks and safety considerations:

  1. Low Blood Sugar: Fasted cardio might be risky if you’re working out for a long time or at a high intensity due to the possibility of low blood sugar. This could lead to lightheadedness, dizziness, shaking, or even passing out.
  2. Dehydration: Dehydration is another potential risk of fasted cardio. It’s important to ensure you’re properly hydrated before, during, and after your workout.
  3. Overeating: When you know you are going to work out in a fasted state, it is possible to compensate by eating more at other times.

Guidelines for Fasted Cardio

Here are some general guidelines on how to safely and effectively do fasted cardio:

  1. Intensity and Duration: Stick to mostly low to moderate-intensity cardio. A 30-45 minute, moderate-intensity workout is recommended. If you’re just starting out, you might want to begin with 20-30 minutes of fasted exercise and then increase intensity and duration if you feel good doing so.
  2. Type of Exercise: Incline walking, light cycling, stairmaster, and so on are all good examples. Choose an exercise that you enjoy and can sustain for the duration of the workout.
  3. Listen to Your Body: Avoid pushing too hard to the point where you feel lightheaded or famished. It’s important to listen to your body and adjust your workout accordingly.

Post-Workout Nutrition

Refuel your workout with protein and carbs within an hour for optimal recovery.

This is crucial for replenishing your glycogen stores and promoting muscle recovery.

Here are some meal options:

Eggs and Whole Wheat Toast:

  • Beat 4 to 5 large eggs with kosher salt, paprika, and red pepper flakes.
  • Add chopped bell pepper, cherry tomatoes, green onions, crumbled feta cheese, chopped parsley, and a drizzle of extra virgin olive oil.
  • Brush a sheet pan lightly with olive oil and arrange 4 slices of bread in it.
  • Spoon the egg and vegetable mixture onto the bread slices.
  • Bake at 375°F for about 15 minutes.
Total Fat:13.9g
Nutritional values

Oatmeal and Yogurt:

  • In a large bowl, mix together oats, yogurt, and milk.
  • Cover and refrigerate overnight, for at least 8 hours.
  • In the morning, give everything a good stir. If it is too thick, add a bit more milk or yogurt.
  • Top with your favorite toppings.
Total Fat:12g
Nutritional values

Chicken and Rice:

  • In a large skillet, cook chicken thighs on both sides until golden. Then remove from the pan and set aside.
  • Saute onions, spinach, and garlic with oregano, salt, pepper, and the reserved marinade.
  • Add rice and broth to the skillet and bring it to a simmer.
  • Place the chicken thighs on top, cover the skillet, and bake for about 35 minutes.
  • Then, remove the lid and bake for an additional 10 minutes.
Total Fat:7.1g
Nutritional values

Salmon with Quinoa:

  • Season and rub a little oil all over the salmon fillets, then cook on a hot griddle for 3 to 4 minutes on each side, or until the fish is cooked through.
  • Pile the quinoa on a plate and arrange the griddled courgette and lemony fennel on top, along with flakes of salmon.
  • Dot over the yoghurty dressing, scatter over the remaining herbs, then serve.
Total Fat:22.4g
Nutritional values


While fasted cardio could potentially increase fat oxidation during exercise, the overall effectiveness of this practice for weight loss and fat burning is still under debate.

However, the scientific evidence on its effectiveness for fat loss is inconclusive.

While some studies suggest a slight fat-burning edge, others show negligible differences compared to fed workouts.

Additionally, concerns exist about potential negative impacts on performance, muscle loss, and hormonal shifts, particularly for intense or prolonged exercise.

The key takeaway is that a balanced diet and regular exercise are still the most important factors for losing weight and improving your health.

Before you decide to incorporate fasted cardio into your fitness regimen, it’s crucial to consider your personal health goals, lifestyle, and any potential risks.

Always remember to consult with a healthcare or fitness professional before starting any new fitness regimen.


Hi there, I'm Pranay, a fitness enthusiast who loves working out regularly and staying in shape. I'm passionate about health and fitness, and I'm always on the lookout for new and exciting ways to stay active and healthy.

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